Reaction of 1-amino- and 1-chlorophthalazine with mammalian molybdenum hydroxylases in vitro.

1-Amino- and 1-chlorophthalazine were tested for possible substrate activity with partially purified rabbit-liver aldehyde oxidase and bovine-milk xanthine oxidase. 1-Chlorophthalazine was a more efficient substrate than the parent compound, phthalazine, with either aldehyde oxidase or xanthine oxidase. The oxidation product of 1-chlorophthalazine was identified as 4-chloro-1-(2H)-phthalazinone on the basis of chromatographic, infra-red and mass-spectral data. 1-Aminophthalazine was oxidized by aldehyde oxidase to 4-amino-1-(2H)-phthalazinone but was a competitive inhibitor of xanthine oxidase. Kinetic studies at different pH values indicated that, in each case, it is the unprotonated form of 1-aminophthalazine that reacts with the molybdenum hydroxylases.